Brittle-bronze, sometimes called by its German name, "glocken-messing" (bell-bronze) is an extremely hard and very dense alloy. Because of its composition, it is able to effectively store any energy that is transmitted to it, which is why--for centuries--it has been used as a material for the manufacture of bells.

A great deal of the energy that is generated by the striking or
plucking of an acoustic guitar string is dissipated at the bridge and
therefore wasted. The more solidly-built electric guitar utilizes its
string energy much more effeciently than the lightly-constructed bridge
area of even the finest acoustic guitar. However the John Pearse
KingPins™ eliminate much of that energy loss, acting almost like small
energy 'batteries' at the bridge-end of each string, allowing a more
efficient use of the string energy--and greatly improving both sustain
and tone.